The demand for increased bandwidth in fiberoptic telecommunications has driven the development of semiconductor transmitter lasers usable for propagation multiple data streams concurrently in a single optical fiber. In addition to telecommunication applications, semiconductor lasers are now commonly used within audio, visual and personal computer systems where they are employed in the read and, where applicable, write heads of CD, CDROM and DVD units.
Usually, semiconductor lasers operate with a wavelength spectrum consisting of a group of several closely spaced wavelengths. However, many applications require the single wavelength operation, narrow linewidth characteristics. This can be achieved by using a wavelength selective filter. One example of a commonly used wavelength selective filter is an etched grating incorporated within a structure to form Distributed Bragg Reflector (DBR) and Distributed Feedback (DFB) laser. However, statistical variation associated with the manufacture of an individual DBR and DFB laser results in distribution of the center of the fixed wavelength. To solve this problem, the DBR and DFB lasers are augmented by external reference etalons and usually require feedback control loops.
Conventional single frequency lasers may also be made to be tunable over wavelength ranges from one nanometer to several tens of nanometers. In optical networks, tunable lasers offer many compelling advantages over fixed wavelength devices. This is due to the fact that tunable lasers simplify planning, reduce inventories, allow dynamic wavelength provisioning, and simplify network control software, making the tunable lasers suitable for use in wavelength-agile applications, for example for wavelength sparing in wavelength division multiplex (WDM) systems.
A tunable laser can be realized as monolithic or hybrid integration. In the monolithic integration, all the components are implemented in a semiconductor substrate. In the hybrid integration, a laser cavity is implemented in a semiconductor chip, while spectrally sensitive elements (external wavelength selective filters) are implemented in a different optical medium.
A typical tunable laser is composed of an optical cavity that encompasses a gain section, and a tunable wavelength selective filter. The gain section includes a medium which can, for example, be a semiconductor based structure utilizing an active semiconductor material, e.g. a composition which is selected from InP, InGaAsP, GaAs, InGaAs, AlGaAs, InAlGaAs. In turn, the tunable wavelength selective filter can be realized as a micro-resonator, waveguide grating, fiber grating or bulk grating. These features are described, for example, in the following publications: B. Pezeshki, “Optics & Photonics News,” May 2001, p. 34–38; WO 00/24095; WO 00/49689; WO 00/76039; and WO 02/31933. In particular, when a laser utilizes a wavelength selective filter in the form of a grating, the laser tuning can be performed by the modification of the grating, for example, by applying an external field (such as heat, stress, etc.) or by means of free careers injection (electron plasma).
Micro-ring resonators can provide high quality tunable wavelength selective filters. A laser constructed in a ring structure is disclosed in WO02/21650, assigned to the assignee of the present application. Such a laser is unidirectional in its operation. This is associated with the unidirectional nature of a ring resonator: light coupled into the ring at a coupling region propagates in the ring only in one direction. The use of ring resonators in a laser thus presents a problem in designing the laser cavity.
Light coupling into a ring resonator to provide light circulation in opposite directions around the ring has been proposed (e.g., U.S. Pat. No. 5,420,684) for creating a resonant interferometer. According to this technique, a passive resonator gyroscope is provided, in which light from a coherent or broadband source is injected into a waveguide beam splitter is coupled to a fiber optic ring. Frequency modulated light of substantially equal intensity circulates in opposite directions around the ring, and the returning light beams are recombined into the original waveguide with a portion of the recombined light provided to a photodetector. Due to the rotation of the ring, a Sagnac frequency shift is produced.